When polyvinyl chloride, antimony trioxide, and zinc oxide are added to a low density flexible and semiflexible polyetherurethane flame retardant foam formulation including the usual tertiary amines, water, and metal containing catalysts and the formulation is run on production equipment as commercially used to form buns (loaves or slabs) having, for example, a cross section of about 30 .times. 76 inches and a length of about 40 feet or more, the foam discolors (dark pin or purple) in the center section of the bun as to be virtually unsalable as is commercially.
The only external heat applied to the bun as it moves down the conveyor is that received when it passes through an oven at a speed of about 5 to 12 feet/min. and at a temperature of about 180.degree.F. for the purpose of reducing or eliminating surface tack so that the outside paper can readily be removed without tearing large chunks from the foam.
At the end of the conveyor the buns are removed and stored for about 24 hours without any external heat being applied to permit the foam to finally cure or stabilize by itself. Then the foam is slit into mattress size units or into other sizes for further manufacturing operations. Even at this time the temperature of the center of the bun is well above room temperature.
The exothermic heat of reaction of the foam forming ingredients during foaming reaches temperatures of from about 270.degree. to 350.degree.F. These temperatures are determined by a pyrometer probe at the end of the conveyor line and can be higher than shown considering that the mass of the pyrometer is greater than that of the surrounding foam.